How to prepare a knife blank
This post will answer the question of how to prepare a knife blank from recycled blades. If you already have clean tool steel to start from you can skip ahead to how to make a wood carving knife.
The first and most important step is picking the right steel. What we are looking for is a tool steel or high carbon steel so it can be hardened.
If recycling saw blades, make sure to pick one where the teeth are made from the same steel as the body. If the teeth are carbide brazed on or HSS welded on (bimetal blade) the steel of the body will be soft and likely not of suitable quality.
The best sources are mill bandsaw and circular resaw blades where the teeth are forged from the steel of the blade body. Reciprocating saw blades can also be used.
Files can also be used but tend to start off very thick and require a lot of grinding to get a flat surface to start with. Car and truck leaf springs can be used for very thick tools like adzes and axes but are not recommended for knives due to the massive amount of grinding needed.
Second step is removing loose rust from the saw blade. I highly recommend a knotted wire wheel for an angle grinder for this. They typically run $20CAD+ but are worthwhile. Get the radial wire wheel not the cup brush. Wear breathing protection from the rust dust and impact protection from wires breaking free. Apron and full face respirator is recommended. Always run the wheel so the rotation goes off edge of the steel so it does not snag.
It should only take a few minutes to clean a very large piece of steel. Getting rid of excess rust now just makes it easier to work with later.
Cutting tool steel
Next the blade must be cut into blanks. As blade steel is hardened I recommend cutoff wheels for angle grinders. Again wear breathing protection from the grinding dust and impact protection from shattered cut off wheels like an apron and full face respirator.
Draw a line with a sharpie marker before cutting and use a bright light to ensure you can see the line while cutting. Sharpie brand markers are very good at drawing on steel while off brands of marker often don't work at all on steel.
Sanding tool steel
A large grinder like a 2x72 or 6x48 is good for flattening the sides of the knife blank and straightening the blank. 4x36 grinders can work in a pinch but have little torque and can stall during this task when applying enough pressure to use ceramic belts to their full potential.
6x48 belt selection can be rather poor due to it being considered a wood sander, but trugrit.com sells a number of high quality belts for it and many other sizes of grinder. 2x48 belts have a much wider selection and also work on a 6x48 grinder.
Sanding Belt Selection and usage
Use a high quality ceramic belt like Norton Blaze for optimal belt life on a super high surface pressure job like this. They are expensive but last a very long time and cut very quickly. Ceramic belts will last longest with high pressure and speed.
Apply pressure till your sander starts to stall, then back off slightly will make a ceramic belt last the longest for amount of metal removed. Too little pressure and the ceramic belt will dull instead of self sharpen.
Generally I will cut a blank suitable for making two knife blades when making small wood carving knives, as a 10"+ long blank is easier to handle at times then a 5"~6" one. The knife blade is cut into two after final sharpening.
Once you are done making the knife blank reasonably straight you can proceed to annealing the knife blank to make it easier for further grinding steps and bending.